Radio-frequency self-diagnosis is a method commonly used in radio systems, where the transmitter and receiver are connected to a common antenna via a duplex filter. Self-diagnosis is effected by having the transmitter send a test signal on the transmission frequency which is then converted into the reception frequency and fed into the receiver. The test signal may contain a known bit pattern that can be used to infer whether the transmitter and receiver are operating error-free. For the diagnosis, a separate test loop connecting the transmitter to the receiver may be used, or the test loop may include parts of the transmitter and receiver branch signal path.
FIG. 1 shows one known circuit for carrying out the test. This type of connection and its variations have been described in patents, such as U.S. Pat. No. 5,337,316, Weiss et al. The transceiver unit includes the transmitter 100, connected by the cable 101 to the duplex filter 102, as well as the receiver 103, connected to the duplex filter by the cable 104. The transmitter and receiver may be tuned to different frequencies, therefore their local oscillator frequencies are generated using the frequency synthesizer. The foregoing represents basic expertise in the field. The test loop is formed by having TX coupling 105 sample the test signal generated by the transmitter and fed to the transmitter branch, typically containing a known bit pattern. The coupling 105 may be a directional coupler that is used to sample the desired output power from the transmitter output power. The test signal is led through the switch 107 to the mixer 106, where the frequency is converted from the transmission frequency into the reception frequency. Then, the frequency-converted signal passes to the directional coupler 106 in the reception branch, to be relayed to the receiver 103. Thus, the test loop consists of the transmitter, directional coupler 107, switch 105, mixer 108, directional coupler 106, and the receiver.
One embodiment of the circuit in accordance with the said US patent is that the transmitter's transmission frequency and the receiver's reception frequency are set to an identical level. Then, the mixer in the test loop can be omitted and replaced by a switch that passes the test signal to the receiver during the test.
Testing is controlled by the control unit 111 which controls the transmitter and receiver frequencies, as well as the voltage-controlled oscillator 106 which generates the mixing frequency. The control unit also controls the switches 107 and 109 that are “closed” during the test. Additionally, the control unit may generate the test signal contents and analyze the test signal received by the receiver.
FIG. 2 shows the principle viewed at the frequency level. The figure depicts the transmission band TX and the reception band RX of a particular system. The band is divided into channels (not shown). When the transmitter generates the test signal on a channel using the transmission band TX frequency fT(x), the transmission frequency is converted into the reception bank RX frequency fR(x) by the mixer assembled on the test loop. Thus, the test signal travels within the transceiver unit. Preferably, the difference in transmission and reception frequencies is identical to the duplex range, but this is not necessary if the control unit controls the synthesizers independently of each other.
In the test procedure described above, part of the transmitter and receiver branch is included in the test loop.
In U.S. Pat. No. 5,457,812, Siira et al. provide a description of testing using a separate test loop. The principle of this solution is shown in FIG. 3. The system depicted here could be a base station in a cellular network with several transceiver units, six in this instance. The transmitters are designated as Transmitter 1 through Transmitter 6 and the receivers in a similar fashion as Receiver 1 through Receiver 6. In normal operation, the radio signals from the transmitter outputs are combined by combiner 310 into a combined signal that is sent along the shared cable to the duplex filter 311 and further on to the antenna. The received signal is led from the antenna to the duplex filter 311, from where the sum signal is led to the reception branch, amplified by the amplifier 312 and finally divided by the wideband divider 313 for delivery to the individual receivers. Thus, all the frequencies are equally divided between all receivers and each receiver selects its own frequency by means of the mixer and narrow-band filter. This is a solution commonly known in prior art.
The correct operation of the transmitters and receivers in accordance with the said US patent is verified so that each transmitter has a separate output for the test signal. In FIG. 3, these outputs are denoted by references a through f. Each output has two output impedance modes: a low-impedance mode of approx. 50 Ω when the transmitter is transmitting the test signal and a high-impedance mode of approx. 10 kΩ when no test signal is being transmitted. The outputs are connected to the shared test cable 314 in such a way that the distance between the connection points of the test outputs of two adjacent transmitters on the shared cable is λ/2 or any multiple thereof. Thus, the distance between the connection points of the test outputs of transmitters 1 and 2 on the test cable is n●λ/2, that of the connection points of the test outputs of transmitters 2 and 3 n●λ/2, and so on. The test cable is connected to the loop unit 315 that carries out the frequency conversion.
The transmitters send their test signals one by one. The test output of the transmitting transmitter is in the low-impedance mode while the others are in the high-impedance mode. The non-transmitting transmitters connected to the test cable using the half-wave technology are visible to one another as practically infinite impedances, and so will not impose any load on one another. The only load that the loop unit 315 performing the frequency conversion is subjected to is the low impedance of the transmitting transmitter. Consequently, a single loop cable is enough and it is not necessary to use separate cables to connect the outputs of each transmitter to the loop unit 315, which would then have to include a combiner feature as well.
These prior-art transceiver test procedures have certain drawbacks. Solutions of the type shown in FIG. 1 require an extra synthesizer to generate the mixing frequency, a mixer stage, signal switching circuits such as directional couplers and switches to minimize the effect of testing on normal operation. Even if the transmitter and receiver used the same frequency, control switches would still be necessary. A synthesizer needs to be programmed to ensure that it operates on the right frequency and the switches have to be controlled as well. When a solution of the type shown in FIG. 1 is used in an environment where there are several transceiver units, not only a large number of cables, but also a large number of switches, mixers, and oscillators are required. The number of components can be reduced by employing the known solution shown in FIG. 3. Moreover, the test can only be carried out within a single transceiver unit (TRX), making it impossible to say whether it is the transmitter or the receiver that is defective.
A major problem with prior-art solutions is that they are only capable of testing the performance of the transmitter and receiver. They cannot be used for testing the correct operation of the duplex filter or even detecting if the filter is missing or if the cable connecting the transmitter and receiver to the filter is defective or completely missing.